This continuation application examines the dual proposition that two enzymatic reactions, i.e., transglutaminase-mediated cross-linking and pepstatin-inhibitable proteolysis, both of which are turned on by an elevation of intra-cellular concentration of Ca++, have profound consequences for the structural organization of membrane proteins and, hence, the physical properties of the red cell. Specific aims relate to basic research as well as to pathological applications and are subdivided into the following categories: studies with human erythrocyte transglutaminase; compositional analysis of the Gamma-glutamyl-Epsilon-lysine cross-linked polymer formed in the membranes of Ca++ enriched cells; dynamics of forming Gamma-glutamyl-Epsilon-lysine cross-linked polymeric clusters and analysis of cross-link frequency; mode of action of pepstatin on the human red blood cell; evaluation of novel non-competitive inhibitors of red blood cell transglutaminase; differential examination of the effects of the enzymatic reactions which cause cross-linking by Gamma-glutamyl-Epsilon-lysine bridges and proteolysis in the membranes of Ca++ enriched red cells, from the point of view of irreversible fixation of echinocytic shape, permanent loss of membrane deformabiity and change in membrane density; facilitation of red cell phagocytosis by the Ca++ induced and pepstatin inhibitable degradation of band 3; measurement of the Gamma-glutamyl-Epsilon-lysine cross-link frequency in polymers obtained from the membranes of irreversibly sickled cells and evaluation of band 3 and glycophorin breakdown; transglutaminase-mediated cross-linking and pepstatin-sensitive proteolytic reactions in red cells in relation to malaria infection.